It's just completely different. As someone who was an outfielder, albeit just at a high school and low-end college level, 98%+ of the running in a game is on defense. You might be on base once or twice a game and have to run (hopefully!) As much as 360 feet around, usually in a line or with a couple of pretty predictable turns in the middle. You run way more than that just running out to your position and back every inning on defense. Then, you have to be backing up the other outfielders on fly balls not just ones hit to yourself, and backing up the infielders on all the plays on the infield. You're basically running on every play quite a bit if you're doing a good job. And, it's much less predictable: you might have to move any direction at any given speed at any moment. It's quite a lot more dynamic stress on the joints and muscles. I 100% get where he could get a good burst a couple times a game on the bases but not be able to play the outfield. It's a huge leap up in work and stress and strain.